The present invention relates to a novel semiconductor plastic package having a structure in which at least one semiconductor chip is mounted on a small-sized printed wiring board, and a method of producing a novel printed wiring board for a semiconductor plastic package. The printed wiring board has a metal sheet and is used for the above semiconductor plastic package. More specifically, the present invention relates to a method of producing a printed wiring board having a structure in which a via hole is, or via holes are, formed so as to be connected to an opposite surface or to front and opposite surfaces of a metal sheet, and the via hole is, or the via holes are, filled with, or the entire wall of the via hole(s) is coated with, a metal plating. The above printed wiring board is suitable for a relatively high-watt-consuming, multi-terminal-possessing and high-density semiconductor plastic package, i.e., for a microprocessor, a microcontroller, ASIC, graphic or the like. The semiconductor plastic package of the present invention is mounted on a mother board printed wiring board with solder balls, and the resultant unit is used as an electronic unit.
As a semiconductor plastic package, there is known a semiconductor plastic package having a structure in which a semiconductor chip is fixed on the upper surface of a plastic printed wiring board. The chip is bonded to a conductive circuit formed on the printed wiring board by wire bonding. Conductive pads for connection to a mother board are formed on the opposite surface of the printed wiring board with solder balls, conductive circuits on front and opposite surfaces are connected with a plated through hole, and the semiconductor chip is encapsulated with a resin. The semiconductor plastic package is known as a plastic ball grid array (P-BGA) or plastic land grid array (P-LGA). In the above known structure, a plated through hole for heat diffusion is formed, which plated through hole is connected from an upper-surface metal foil, on which a semiconductor chip is fixed, to a lower surface for diffusing heat generated by the semiconductor to the mother board printed wiring board.
Water is absorbed through the above through hole into a silver-powder-containing resin adhesive used for fixing the semiconductor. During heating, when a semiconductor part is mounted on the mother board or during heating when a semiconductor part is removed from the mother board, an interlayer swelling may take place, which is called a popcorn phenomenon. When the popcorn phenomenon takes place, the package is no longer usable in many cases, and thus it is desired to decrease the above phenomenon drastically. Further, a higher function and a higher density in a semiconductor results in an increase in the amount of heat to be generated, and the formation of only a through hole directly below the semiconductor chip is insufficient for heat diffusion.
The present invention therefore provides a semiconductor plastic package in which the water absorption from a reverse surface is prevented, and which is remarkably improved with regard to heat resistance after water absorption, i.e., drastically improved with regard to the popcorn phenomenon, excellent in heat diffusion, suitable for mass production and improved in economic performance. A method of producing a printed wiring board for use in the above plastic package is also provided. The present invention further provides a semiconductor plastic package which is excellent in electric insulation and migration resistance after subjected to a pressure cooker test, and a method of producing a printed wiring board.
According to the present invention, there is provided a semiconductor plastic package having a structure in which a metal sheet having a size nearly equivalent to a printed wiring board is disposed nearly in the central portion in the thickness direction of the printed wiring board. At least one semiconductor chip is fixed on one surface of the printed wiring board with a thermally conductive adhesive. The metal sheet and a signal propagation conductive circuit on the front surface are insulated from each other by a thermosetting resin composition. The semiconductor chip is connected to the conductive circuit formed on the printed wiring board surface by wire bonding. At least the conductive circuit on the printed wiring board surface is connected to a conductive circuit formed on the opposite surface of the printed wiring board, or to a conductive circuit pad formed, for being connected to an outside of the package with solder balls, with a conductive through hole insulated from the metal sheet via a resin composition. And, at least the semiconductor chip, the wire and the bonding pad are encapsulated with a resin.
The semiconductor plastic package has at least one via hole made in the opposite surface so as to be directly connected to the metal sheet, with the via hole having an inner wall rendered thermally conductive. The printed wiring board is provided, on the semiconductor-chip-mounting side, with an elevated portion, a plurality of protrusions each having the form of a frustum of a pyramid or a cone, or with a via hole having an inner wall rendered thermally conductive.
Further, according to the present invention, there is provided a semiconductor plastic package according to the above invention, wherein a plurality of the protrusions each having the form of a frustum of a cone each are in contact with the opposite surface of a copper foil, to the front surface of which the semiconductor chip is directly fixed.
According to the present invention, further, there is provided a semiconductor plastic package according to the above invention, wherein the at least one via hole is formed on the semiconductor-chip-mounting side directly below the semiconductor chip so as to be directly connected to the metal sheet.
According to the present invention, further, there is provided a semiconductor plastic package according to the above invention, wherein the metal sheet is formed of a copper alloy having a copper content of at least 95% by weight or pure copper.
According to the present invention, further, there is provided a semiconductor plastic package according to the above invention, wherein the thermosetting resin composition contains a polyfunctional cyanate ester or a prepolymer of the cyanate ester.
According to the present invention, there is also provided a method of producing a metal-sheet-possessing printed wiring board having a structure in which a metal sheet having a size nearly equivalent to a printed wiring board is disposed nearly in the central portion in the thickness direction of the printed wiring board. The metal sheet is insulated from conductive circuits on front and reverse surfaces with a thermosetting resin composition, and the conductive circuits on the front and reverse surfaces are connected to each other with a conductive through hole insulated with a thermosetting resin composition.
The method comprises forming a clearance hole or a slit hole in the metal sheet and in a position where the through hole is to be formed, providing each surface of the metal sheet with at least one of a prepreg, a resin sheet, a coating and a resin-applied metal foil, and placing a metal foil on any outer resin layer if no metal foil is present. The method further comprises laminate-forming the resultant set under heat and under pressure to provide an integrated metal-sheet-possessing copper-clad laminate, and to fill the clearance hole or the slit hole with a resin composition of the prepreg, the resin sheet, the coating or the resin-applied metal foil. The method further comprises, then making a penetration hole for the through hole so as to be out of contact with the metal sheet, making a via hole for heat diffusion in the opposite surface so as to be connected to the metal sheet, filling the via hole portion with a metal by metal plating, and electrically connecting the front and opposite surfaces by metal plating of the through hole. The method further comprises, forming circuits on the metal foils which are the front and opposite surfaces, coating any portion other than the semiconductor-chip-mounting portion, a bonding pad portion and a ball pad portion with a plating resist, and then carrying out noble metal plating.
According to the present invention, further, there is provided a method according to the above invention, wherein the semiconductor-chip-mounting side surface of the metal sheet has an elevated portion having the form of a frustum of a pyramid or a cone.
According to the present invention, further, there is provided a method according to the above invention, wherein a via hole for heat diffusion is also made on the semiconductor-chip-mounting side. The via hole is provided immediately below the semiconductor chip so as to be connected to the metal sheet, and the via hole is filled with the metal by performing metal plating.